1. Field of the Invention
The present invention relates to a method for nickel concentration processing of a saprolite ore, for more detail, to a method for nickel concentration processing from a low grade saprolite ore which is not used for ferronickel smelting process, with low cost, simple and convenient, as well as having low environmental load. By using this method, resource amount of a nickel ore, now approaching to a nearly depleted state, can be increased, transportation cost and smelting cost can be reduced, and still more prevention of environmental problems can be attained.
2. Description of the Prior Art
In general, a raw material ore for nickel smelting is largely classified to a sulfide ore and an oxide ore such as a laterite ore. And in addition, the laterite ore is classified to a saprolite ore and limonite ore. The saprolite ore has a relatively high nickel content of equal to or higher than 2% by mass, containing magnesia, silica, iron and the like as main composition components, and composed of ores such as a water-containing silicic bittern ore, goethite. The limonite ore present at the upper part thereof, having a low nickel content of about 1.5% by mass, and composed of goethite as a main ore.
The above saprolite ore has been used from old times, as a practical raw material ore to produce ferronickel, however, the saprolite with high nickel content has been depleted, and nickel content of a raw material ore to be utilized in ferronickel smelting has been decreased, which has raised a large problem in view of economical production.
That is, in ferronickel smelting, usually, a saprolite ore containing a large quantity of moisture is subjected to roasting at a high temperature of up to about 900° C., in order to decrease an attached moisture content and a crystal moisture content to a predetermined level, and then the resulting calcinated ore is subjected to reductive dissolution in a melting furnace such as an electric furnace at a temperature of about 1500° C., so as to produce ferronickel with predetermined nickel content satisfying a product standard. Therefore, decrease in nickel content of a raw material ore not only increases consumption amount of energy such as electricity, heavy oil, and increases smelting cost extremely, but also could decrease the resulting nickel content of ferronickel produced in the electric furnace, to a level below the product standard required on the market, although it depends on a containing state of iron present together. Therefore, it also contains an environmental problem.
Furthermore, special steel based mainly on stainless steel occupies a large portion of nickel consumption, therefore it is important to secure amount of ferronickel, and stable supply of a saprolite ore with high nickel content now depleting at present, can be said an urgent problem. Incidentally, a wet-type smelting method such as a sulfuric acid leaching method of a laterite ore, which has been progressing recently, is generally suitable to a limonite ore with low magnesium content, however, on the contrary, it is not necessarily suitable to a saprolite ore with high magnesium content, in view of high acid consumption etc. In addition, a saprolite ore imported from a mine is usually in a state of having a high moisture content of equal to or higher than 30% by mass, and also a low nickel content of 2 to 2.6% by mass, therefore cost of a raw material ore including transportation cost was increased to a very high level.
Therefore, there has been required conventionally to enhance nickel content of such a laterite ore, and for example, trials have been made to attain the increase in content by application of a beneficiation method such as flotation, magnetic separation (for example, refer to Non-Patent Literature 1, 2). However, these methods have many problems in view of difference in result by each of a target ore, or processing cost, and thus practical application has not yet been achieved.
Under these circumstances, there has been disclosed a method for classification performing of a raw material saprolite ore, and still more specific gravity separation by each of the classified portions (for example, refer to Patent Literature 1, 2, 3, 4). However, these methods are wet-type methods, and provide ores with very poor precipitation property and dehydration property, therefore require a large quantity of thickener and a dehydration machine as a facility therefor, and not only increases cost extremely but also requires many labor in environmental preservation such as exhaust water processing, management of a tailing dam. Therefore, a method for concentration of slime using an organic flocculant has also been proposed (refer to Patent Literature 5), however, it requires a large quantity of the flocculant and thus has not led to cost reduction. Still more, since high water content of a raw material is equal to or higher than 30% by mass, in order to reduce transportation cost and smelting cost, drying of an ore at a mine site was tried, however, it has found to create a new problem of nickel amount loss and deterioration of workability, caused by scattering in handling such as loading and unloading or the like, because the ore has very strong powder dust property in the case of low water content, and thus practical application is in a difficult state.
Under such a state, it has been required a method for nickel concentration processing, which is low cost, simple and convenient, as well as having low environmental load, which is thus capable of enhancing nickel content of a saprolite ore with low nickel content, to a level to be utilized economically as a raw material of ferronickel smelting.    Patent Literature 1: U.S. Pat. No. 6,053,327    Patent Literature 2: JP-A-52-023504 (page 1)    Patent Literature 3: JP-B-03-004610 (page 1)    Patent Literature 4: JP-A-11-117030 (page 1, page 2)    Patent Literature 5: JP-A-11-124640 (page 1, page 2)    Non-Patent Literature 1: “Lecture summary of research result presentation by The Mining Institute of Japan”, 1987, pages 365 to 366    Non-Patent Literature 2: “CIM Bull”, (Canada), vol. 93, No. 1038, 2000, pages 37 to 43